The present invention relates to a root post with a fixation part made from precious-metal or base-metal alloys that can be inserted into the curved root canal of a dental root.
A natural human tooth consists of a crown and a dental root. The crown, which makes up about 1/3 of the total length of the tooth, projects visibly from the gums into the oral cavity. The conically shaped dental root, about 2/3 of the total length of the tooth, is concealed and fixed in jaw ridge. The nerve of the tooth is embedded in the root from the tip of the root to about the middle of the crown in an axial, conical, curved canal.
In most cases, when the crown is destroyed, e.g., due to caries or trauma, the root, after removal of the nerve, can still be utilized for a prosthetic appliance in order to close the gap between the teeth.
For this purpose, after a well-planned resection of the destroyed crown, the root canal is drilled open with a conical or cylindrical root-canal reamer. Thereafter, by means of a manually operated root-canal trimmer the excavation is brought to a predetermined level that corresponds to the shape and dimensions of a root post to be inserted into the root canal.
Root posts as described for the production of artificial dental structures consist of an elongated, metallic fixation part that serves for the retention in the root, and a head portion mounted at the upper end of the fixation part and on which the new crown is built up. German OLS 35 40 188 shows such a root post.
The fixation or retention part of these root posts is shaped conically or cylindrically and has a smooth surface or is provided with a thread or with structures. As a rule, one uses as materials precious-metal alloys of the AgPd and AuPtPd type and base-metal alloys of the TiAlFe or TiAlv type and CoCrMo.
The root posts are each made of only one material with the identical mechanical properties throughout the length thereof.
All root posts or the fixation parts thereof have a rotationally symmetrical design, since the root-canal reamers and root-canal trimmer too must have conically or cylindrically axially symmetrical shapes and the bores are drilled accordingly in the root canal. The fixation of the posts in the root canals, apart from a certain clamping effect of the post in the canal, is effected via square threads, frictional snug fit, or by cementing them in with phosphate cement.
With roots that have a conical shape and are generally curved, drilling must be effected in a conically or cylindrically axially symmetrical fashion. With these bores, one can at least make allowance for the anatomical conditions of the root and one cannot rule out the danger that during the drilling in a root curvature the root will be perforated. Moreover, a straight fixation part, which in a curved root is partly implanted outside the root canal as well, can transmit the loads non-physiologically from the crown to the residual root, which may lead to possible damage to the periodentium. Peak stresses in the areas of the root in which the fixation part lies outside the root canal are unavoidable in the event of loads, so that the root can be destroyed.
With masticatory loads, the root post is stressed on bending, so that a fixation part that does not abut properly on the wall of the root can be destroyed by alternating bending stresses. The destruction can occur very quickly if upon insertion of the root post with alignment a relatively thick cement layer is produced which can be pulverized if the root post is subject to bending loads on the root post and which clears a fairly large bending path for the post at the oral outlet of the root canal.
After improper guidance of the post at the oral exit of the root canal, all of the masticatory and bending forces are transmitted to the apical zone where the root apex sits relatively securely. In the apical zone, the root post does not usually sit by analogy with the course of the natural root canal and with the taperingly curved root because of the anatomical shapes of the teeth. The root can be subject to rupture in these cases as well.